In the prior art, shift-register operation has been achieved in semiconductor devices by means of the electrically controlled shifting of localized accumulations of charges (charge packets) in a semiconductor medium. Such charge packets are controllably translated through the semiconductor by means of applied clock voltage pulses which transfer electrical charges from one storage site to the next in the semiconductor device. Thus, such a semiconductor shift-register is, in effect, a type of charge transfer device (CTD). These devices are useful in such applications as delay lines and optical imaging apparatus.
Charge transfer devices in the semiconductor art fall into two main categories, the so-called "charge-coupled device" (CCD) and the integrated circuit versions of the bucket-brigade device (BBD). In either category, a spatially periodic electrode metallization pattern on a major surface of a semiconductor body coated with electrically insulating oxide defines a sequence of integrated MOS (metal-oxide-semiconductor) type capacitor regions, serving as transfer sites, so that localized electrical charge accumulations (or portions thereof) in the semiconductor, originate in response to an input signal. These charge packets can then be shifted through the semiconductor sequentially between adjacent MOS capacitor regions under the influence of a sequence of (clock) electrical voltage pulses applied to the electrodes. The charge packets, in response to a signal, are initially injected at the input end of a chain of such MOS capacitor regions, in accordance with a stream of digital or analog signal information, for example, in the form of signal-controlled injected charges into a first transfer site of the CTD at the appropriate moments of time relative to the clock voltage pulse sequence. As known in the art, the clock voltage pulse sequence can be three-phase, two-phase, or single-phase, in connection with such CTD's as described for example in, respectively: 49 Bell System Technical Journal (April 1970), pp. 587-593; U.S. Pat. No. 3,651,349, issued to D. Kahng et al. on Mar. 21, 1972; and U.S. Pat. No. 3,796,932, issued to G. F. Amelio et al. on Mar. 12, 1974.
It should be understood, of course, that ordinarily in present-day semiconductor charge transfer devices, the semiconductor medium is silicon and the oxide is silicon dioxide; however, other suitable semiconductor-insulator combinations may be used in general. Thus, in particular, the term "oxide" (and hence "MOS") in connection with CTD's can refer to any such suitable insulator.
In application Ser. No. 395,388, filed in the name of M. F. Tompsett on Sept. 10, 1973, U.S. Pat. No. 3,881,117 an input circuit for injecting charges into a semiconductor transfer device was described in which the charge packet input to the semiconductor charge transfer device could be made substantially independent of the surface characteristics of the semiconductor. In that patent application, an input charge injection circuit was disclosed in which signal-independent input charges from a charge source region (input diode) were periodically transferred to the first transfer site of the CTD, and signal-dependent charges were subsequently transferred from this first CTD transfer site back to the charge source. Thereby, the net input (signal) charge packet in the first transfer site, which is then subsequently transferred to the bulk of the CTD, was substantially insensitive to the variable semiconductor surface channel characteristics of the surface of the semiconductor in the neighborhood of the input diode. However, the net input charge of a given packet, representative of the instantaneous signal, was still somewhat degraded by noise from the clock voltages applied to the first transfer site. Also, the linearity of the ratio of net input signal charge to signal voltage, while substantially improved over prior art, was not as good as desired in certain commercial applications for analog signal processing such as analog delay lines, transversal filters, time compressors and expanders.